fix!: require a working CodePush server URL

[GSTJ]

fix!: require a working CodePush server URL

Summary

Prebuild now requires a self-hosted CodePush server URL for Android and iOS, and stops with an actionable error before it can write config for App Center's retired endpoint.

Details

• Makes the server URL required in the plugin config type and at runtime.
• Writes the validated URL into both native projects every time.
• This is a breaking change for apps still relying on the retired App Center default. They need to point both platforms at a self-hosted server before prebuild will run.
• Covers missing and empty URLs on Android and iOS.

Testing steps

1. From the repository root, run:

   pnpm install --frozen-lockfile
   pnpm run test

2. Confirm every command exits successfully. The test output should show Android and iOS rejecting both missing and empty update server addresses.

[GSTJ]

Packed-tarball proof from 3306d581077c9fe338327c877c7d9c306647da26: Expo SDK 57 prebuild writes the update server URL, deployment key, and bundle hooks on Android and iOS. Missing URL runs for both platforms exit 1 with the App Center retirement error. The same run includes the full local gate and the 2.0.0 release recommendation.